Field of the Invention
The invention relates to a method for operating a cryostat which houses a chamber containing at least one object to be cooled.
Description of the Related Art
A cryostat is described in patent application DE 10 2014 218 773, unpublished at the time of filing of the present invention, that comprises a vacuum container having at least one hollow cold head turret which contains a neck tube connecting a chamber containing at least one object to be cooled through the outer shell of the vacuum container to the area outside of the cryostat, the neck tube being geometrically configured in such a way that a cold head may be inserted, the cold head having a cold head stage via which a cryogenic working medium, which flows or drips through a heat pipe into an evaporator chamber during a condensation operation, may be cooled, and which is thermally conductively connected to the object to be cooled via a thermal contact surface, so that the cooled cryogenic working medium is able to absorb heat from the object to be cooled and transport it to the cold head stage via the heat pipe.
In most cases, cryotechnology utilizes cooling machines for cooling objects, for example superconducting magnet coils. The cooling machines discharge heat from the apparatus containing the object to be cooled by means of a cold head.
These cooling machines are typically operated with helium gas as the coolant, which is compressed in a compressor and expanded in the cold head of the cryostat (for example, so-called pulse tube coolers). The cold head and the compressor are generally connected to one another via two pressure lines. The cold head is connected to the components to be cooled, either directly mechanically or via a contact medium (for example, cryo gas or cryo liquid), or in both ways in order to ensure good heat transfer.
However, if the compressor completely or partially fails, for example due to a technical defect or power outage, the previously cooled components heat up. In this situation, the cold head of the cryostat then represents a significant thermal bridge between the components to be cooled and the external surroundings. In the persistent operating mode of a superconducting magnet, the superconducting current can flow practically without resistance for extremely long time periods. However, heating of the magnet causes a so-called quench of the persistent operating mode after a certain time. At some point, the magnet reaches the critical transition temperature, which is predetermined by the superconducting material, and becomes normally conducting and thereby loses its high magnetic field, generally abruptly.
A reduction of the thermal load after failure of the cooling machine would at least considerably extend the time period until a quench occurs. This is true in particular for cryostat configurations that can be operated completely without or with only minimum amounts of liquid coolant; superconducting magnets currently are normally operated in a liquid helium bath. Since helium is becoming increasingly expensive, cryostats that can be operated completely without or at least with minimum amounts of helium (low-loss or even cryo-free systems) are becoming more and more attractive, both technically and economically.
However, the thermal capacity of solids significantly decreases at very low temperatures. For this reason, it would be particularly important for systems of this type, using small amounts of liquid helium or no liquid helium at all, to minimize the heat input into the object to be cooled in case of failure of the cooling unit.
DE 195 33 555 A1 describes a cooling device for indirectly cooling a unit, in particular a superconducting unit, situated in a vacuum housing, and which contains at least one refrigerator portion. This refrigerator portion is composed of a section on the room temperature side and a section on the low-temperature side, protrudes into the vacuum housing, and is fastened thereto via spring elements, and at its end on the low-temperature side is thermally conductively connected to the unit to be cooled. To avoid vibrations being transmitted to the unit, it is provided that the section of the cooling machine portion on the room temperature side is situated in an evacuable space of a housing unit which is fixedly connected to the vacuum housing. This cooling device is thermally conductively connected to the unit to be cooled, so that in the event of failure of the cooling machine, the object to be cooled heats up quickly. When an LTS magnet is used, this means that a quench occurs immediately.
LTS coils are conventionally kept directly in liquid helium, which is very costly due to the fact that helium bath cryostats are very complex and therefore expensive.
DE 10 2011 078 608 A1 discloses a cryostat system comprising a vacuum container and a cryo container installed therein, and a sleeve in which a cryocooler is installed, the upper, warm end of the sleeve being connected to the outer shell, and the lower, cold end facing the cryo container being sealed gas-tight by a sleeve base, and the cryo container containing a superconducting magnet system. The known system is characterized in that the cryo container is hermetically sealed except for a gas capillary, and is filled with gaseous fluid at a pressure below the vapor pressure of the liquid phase of the fluid at the corresponding operating temperature, and the coldest stage of the cryocooler is in good thermally conductive connection with a heat exchanger situated inside the cryo container. In this way, the superconducting magnet system within the cryo container can be cooled without cryogenic liquid, and at the same time, without a direct mechanical coupling to the cryocooler; during the operating period, handling of cryogenic liquids may be dispensed with, and the discharge of cold fluid in the event of a quench of the superconducting magnet system may be avoided.
DE 10 2011 078 608 A1 describes a cryostat without cryogen and without a heat pipe. One disadvantage, however, is that if the cooling unit fails, the user has very little time (“time to quench”) until the magnet heats up and loses its superconducting property. In contrast, a heat pipe can better bridge over a fairly long failure in the cooling, since the cryogen in the heat pipe evaporates first before the cryostat begins to heat up.
The advantages of a cryostat with a heat pipe with regard to the time to quench are described in the patent application DE 10 2014 218 773 cited above. This document describes a cryostat which includes a vacuum container and a chamber containing the object to be cooled, and includes a neck tube which houses a cooling arm of a cold head which is connected to a refrigeration device, and which can be brought into thermal contact with a second thermal contact surface on the object via a first thermal contact surface on the cooling arm. The described system is characterized in that the hollow volume between the inner side of the neck tube, the cooling arm, and the object is filled with gas, wherein the internal pressure of the gas pressurizes part of the cooling arm, whereas another part of the cooling arm is pressurized by atmospheric pressure, that the cooling arm can be moved with its first thermal contact surface toward or away from the second thermal contact surface, and that a contact device establishes thermal contact of the first thermal contact surface with the second thermal contact surface when the gas pressure is below a threshold pressure, and the contact device moves the first thermal contact surface away from the second thermal contact surface when the threshold pressure is exceeded, so that the contact surfaces are thermally separated by a gap filled with gas. In this way, in the event of failure of the cooling machine, the thermal load on the object to be cooled is significantly reduced automatically in an operationally reliable manner without intervention by an operator.
However, this type of cryostat requires a relatively long precooling period before it is actually started up. Heat pipes, despite their other advantages, basically are not powerful enough to quickly cool cryostats down to their working temperature, in particular in the event of failure of the cooling unit, described above. For fairly large cryostats containing, for example, an LTS coil weighing several hundred kilograms, a standard heat pipe with 40 W cooling power, with use of a single cryogen, would require several weeks to precool the coil to a working temperature of 4 K.
Even with successive use of different cryogens having different condensation temperatures in the various subareas, within the fairly large temperature difference from the original room temperature down to the very low operating temperature, a precooling time in the range of week is still required; however, this procedure is naturally extremely complicated, in particular material- and labor-intensive.